It is well recognized that exposure to sunlight is absolutely essential for the development and maintenance of a healthy skeleton in humans when dietary sources of vitamin D are not adequate. Radiation of wavelengths less than 316 nm initiates the production of vitamin D in human skin, a deficiency of which is now being recognized as a health problem amongst the elderly both in Europe and the United States. Sunlight contains a small component of the biologically active UVB wavelengths required for cutaneous vitamin D synthesis and for a generally mobile individual, a normal lifestyle provides sufficient sunlight exposure to maintain a healthy vitamin-D status. However, age is known to decrease the amount of vitamin-D precursor in the skin, and this, together with the lack of outdoor activity undertaken by the elderly and infirm, leads to the bone diseases associated with vitamin-D deficiency. To combat this problem, we need to know how much sunlight exposure throughout the year is required for health. The latter quantity is a function not only of time, but also of skin area exposed, season, and latitude. This study will assess the affect of aging on cutaneous vitamin D synthesis in vivo by monitoring the natural UVB exposure and vitamin-D status of volunteers in a well defined population of elderly in a nursing home setting. Comparison of the (a) solar spectrum with the spectrum with the spectra of fluorescent lights and (b) the capacity of fluorescent lighting to promote cutaneous previtamin D3 synthesis will then enable recommendations to be made for the use of solar irradiation as a means of maintaining adequate vitamin-D nutrition for the elderly mobile residents and for the possible development of UVB-enhanced lighting that could be used in hospital wards and nursing homes for infirm patients who are susceptible to vitamin-D deficiency and cannot go outdoors.